Here's a tangent for December...
"December" means "10th month", going back to the Roman Republic. The root "Dec" means 10, as in decimal system, decileter, decimate (killing every 10th man), etc. In Romance languages the words for "ten" are derived from "dec", such as dix in French and diez in Spanish. This is attributed to the influence of Latin, but I think it's more accurate to say it's the Roman influence.
So, why do we call the 12th month, "The Tenth Month"? Because for the ancient Romans there were traditionally ten months in the year, then along came Julius Caesar. When Caesar took control in the 1st century B.C., he wanted a month named after him, so he inserted a month in the summer and named it after himself (July). Instead of deleting one of the existing months, he simply pushed them forward by one. The following months were not renamed, probably due to tradition.
When Caesar's adopted son Octavian became the sole ruler of Rome, he wanted his own month too. He inserted one right after Caesar's month, and named it after his title, Augustus. The months were pushed forward again. September still means "7th month", though it's the 9th month. October means 8th, though it's the 10th month; November means 9th, though it's our 11th month.